Isolation and characterization of temperature-sensitive mutants of cyanophage N-1

Optimal conditions for the induction of temperature-sensitive (ts) mutants of cyanophage N-1 were established after mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). A treatment with MNNG (400 micrograms/ml) for 2 hrs at pH 8.0 induced ts-mutants at a maximum frequency of 1.46 x 10(-3). A characterization of 10 such ts-mutants with regard to adsorption, one-step growth and temperature-shift experiments with Nostoc muscorum as host bacterium led to the identification of temperature-sensitive steps in the phage multiplication at the restrictive temperature (37 degrees C). All the mutants were found to be conditionally lethal at 37 degrees C since they resumed growth upon shifting to 28 degrees C.